In general, the mirror polishing of a semiconductor wafer such as a silicon wafer, a lens, a glass substrate, or some other is divided to rough polishing for a main purpose of adjusting the polished matter in flatness and in-plane evenness, and finish polishing for a main purpose of improving the matter in surface roughness, and removing scratches.
The finish polishing is usually attained by causing a sueded artificial leather made of a soft foamed polyurethane to adhere onto a rotatable table, and rubbing a wafer against the leather while a polishing agent wherein colloidal silica is contained in an aqueous alkali-based solution is supplied thereonto (Patent Document 1).
However, when the polishing layer, which is made of the foamed polyurethane used for finish polishing, is low in flatness, there is caused a problem that tiny undulations are generated in the surface of the matter to be polished. The tiny undulations have been in particular considered to be a problem in the field of recent finish polishing. Thus, it has been intensely required in the market to supply a polishing pad capable of attaining a further decrease of tiny undulations.
The present inventors have suggested a polishing pad comprising a polishing layer comprising a thermoset polyurethane containing, as raw material components, an isocyanate component plus active-hydrogen-containing compounds comprising 1 to 20% by weight of a low-molecular-weight polyol having a functional group number of 3 to 8 and a hydroxyl group value of 400 to 1,830 mg KOH/g and/or a low-molecular-weight polyamine having a functional group number of 3 to 8 and an amine value of 400 to 1,870 mg KOH/g (Patent Document 2). Although this polishing pad is characterized by being excellent in endurance and self-dressability, it has been made evident that the pad has a room for a further improvement about tiny undulations in a matter to be polished.
Patent Document 3 listed up below states that when the microrubber A hardness value of a polishing pad is at least 12 smaller than the Asker C hardness thereof, and the Asker C hardness is 60 or more, the polishing efficiency of the polishing pad can be made compatible with the flattening performance thereof at a high level. However, this document never discloses a specific formulation or composition of a polyurethane foam constituting the polishing pad. Furthermore, the present inventors' investigations have made it evident that only by optimizing the relationship between the microrubber A hardness value of a polishing pad and the Asker C hardness thereof, the polishing pad cannot gain the effect of decreasing tiny undulations as described above.
Patent Document 1: JP-A-2003-37089
Patent Document 2: JP-B1-4237800
Patent Document 3: JP-A-2008-207319